Measurement of substances within body tissues or fluids, such as blood, are commonly carried out for use in diagnosis or in the monitoring of certain medical conditions. In many cases, such as in the measurement of bilirubin in blood serum, a blood sample is taken for analysis. This is commonly done in the assessment of jaundice in new-born babies by taking a blood sample from the baby's heel. However, this procedure can traumatize the baby, can lead to infection (particularly if repeated samples need to be taken to monitor treatment) and does not provide a reliable measurement at high concentrations of bilirubin.
A number of non-invasive optical techniques have been proposed for measurement of substances such as bilirubin. These involve illuminating the subject's skin with one or more wavelengths of light, detecting the light reflected from the skin, or in some cases transmitted through the body tissues, e.g. through a finger, and analyzing the results to measure a spectral characteristic of the reflected or transmitted light caused by the substance to be measured. Such measurements are, however, subject to interference by a number of factors including: skin pigmentation, maturity, the effects of treatment such as phototherapy and the presence of other substances such as blood hemoglobin. In an attempt to overcome these difficulties, complex analyses, usually of several wavelengths, are carried out to try to reduce the effect of the interferences on the desired measurement.
The present invention aims to avoid or significantly reduce such problems.